Circadian clocks have recently become recognized as modulators of a wide array of physiological processes, including glucose homeostasis, blood pressure modulation, and drug metabolism and toxicity. Several drugs, including the anti-hypertensive statins (e.g. Lipitor) and the anti-diabetic drug metformin (e.g. Glucophage), are recommended to be taken at specific times of day. However, the molecular basis for these preferences is not well understood. The underlying hypothesis of this proposal is that the cellular transport and metabolism of therapeutic drugs can be modulated at the transcriptional level by circadian repressors altering the function of nuclear hormone receptors that respond to xenobiotic ligands. Advancing our functional understanding of these receptor and repressor interactions may highlight new therapeutic strategies for treating disease. For example, defining the diurnal regulation of the xenobiotic transcriptional network could enable the prediction of optimal treatment times for existing and novel therapeutic compounds. In addition, a deeper understanding of the diurnal regulation of nuclear hormone receptor pathways that modulate the xenobiotic transcriptional response may lead to new strategies for modulating drug absorption, metabolism and/or toxicities. Our previous studies identified the circadian clock component cryptochromes (Cry1 and Cry2) as nutrient- responsive transcriptional regulators by virtue of their susceptibility to phosphorylation by AMP-activated protein kinase (AMPK) and their ability to modulate glucocorticoid receptor dependent transcription. We have also established an important role for the liver circadian clock in glucose homeostasis, via driving diurnal expression of hepatic enzymes and transporters, including AMPK. In the course of those studies, we have generated unique tools and expertise that enables us to use biochemical, genetic, molecular and physiological approaches to uncover the roles of circadian clocks and of the circadian repressors Cry1 and Cry2 specifically in nuclear hormone receptor pathways governing the control of drug transport and metabolism, in the following specific aims: 1) Characterize the interactions of Cry1 and Cry2 with the xenobiotic receptors PXR and CAR (define the species and isoform specificity and ligand dependence of cryptochrome-xenobiotic receptor interactions; identify domains and sequences required for interaction), 2) Define the role of Cry1 and Cry2 in the regulation of PXR/CAR-mediated xenobiotic metabolism (identify specific transcriptional targets of PXR and CAR that are regulated by cryptochromes, measure changes in drug transport and metabolizing activities upon cryptochrome depletion), 3) Examine the roles of circadian clocks, PXR and CAR in the efficacy of metformin treatment.